Conventionally, the cutting off or termination of the starting phase of the engine, in which the engine is driven by its starter, is controlled by the user of the vehicle, who cuts off the supply to the starter motor by releasing the ignition key when the engine makes the appropriate characteristic sound. However, the current tendency to make internal combustion engines more and more silent results in the fact that it is becoming difficult for the driver to detect such characteristic sound, and therefore to detect that the engine has started. This leads to undue delay in releasing the ignition key, and, as a result, to the application of unnecessary and excessive forces on the starter.
Numerous devices for cutting off the power supply of a motor vehicle starter when the engine has started and is sufficiently autonomous to achieve its slow running mode by itself, are already known. In particular, it has been proposed to control cut-off of a starter by comparing the battery voltage with a reference voltage, the power supply to the starter being cut when the battery voltage reaches this reference voltage. A device which operates in this way is described in the introduction of French patent specification No. FR 2 626 417.
In this connection, it is known that when a starter begins to drive a heat engine, the voltage across the battery (the battery voltage) undergoes a sharp reduction in level due to the heavy current that flows in the starter motor. The battery voltage regains its initial level when the engine has started.
However, such a device is not fully satisfactory. In particular, the comparison with a predetermined reference value does not enable sufficient precision to be obtained in the controlled cut-off of the starter power supply. In addition, the battery voltage can undergo considerable variation over a period of time, in particular as a function of the state of battery maintenance, or as a function of ambient temperature.